The cost of solid-state RF power supplies used for CO2 lasers is approximately equal to the cost of the laser head. It is well known to those skilled in the art that, as the RF laser excitation frequency of the power supply is increased, the amount of RF power that can be coupled into the laser's gas discharge can be increased to a higher value without developing arcs within the discharge volume. Arcs within the discharge are detrimental to CO2 laser performance. It is also well known that, as the RF frequency is increased, the discharge can be operated at higher pressures while still maintaining a uniform discharge. Both of these higher frequency benefits enable higher laser output power to be obtained from a given-size laser head.
Unfortunately, as the RF frequency of the solid-state power supply is increased, the design, assembly and cost of the power supply increases. Laser manufacturers are forced to make compromises between benefits in decreased laser size for a given laser power output, versus the disadvantages associated with design, assembly and higher cost associated with the use of higher-frequency solid-state power supplies.
Typically, frequencies in the VHF band (e.g. 30 to 300 MHz) are utilized in sealed-off, RF excited, diffusion cooled CO2 lasers, with the frequencies between 30 MHz and 100 MHz being most common.
RF power supplies for the above-discussed lasers usually include a master oscillator and at least one stage of amplification. One of the more challenging tasks in designing RF power supplies for CO2 lasers is the matching between the final RF amplifier and the laser discharge. The final RF amplifier may require impedances as low as 5-10 Ohms, while the transmission line carrying RF power to the discharge has an impedance that is typically 50 Ohms. The laser discharge impedance is in the order of 50 Ohms after ignition for laser power of 100 W. The power drops for higher power CO2 lasers. This match must be efficient such that minimum RF power is lost within the impedance transformation. In addition, the impedance of the un-lit discharge is much higher than for the lit discharge. Consequently, there is a large mismatch prior to the discharge being lit. Additionally, the match must be able to withstand the possible high voltages generated during the process of igniting the discharge. Since to light the discharge requires a higher voltage than to keep it running after ignition, the ignition is usually performed with a high voltage pulse or a series of fast pulses. Transmission line transformers are inherently broadband so that they can deliver the high voltage “spikes” and they are also very efficient under continuous wave (CW) operation. Consequently, they are presently the preferred choice for this application. As discussed in detail below, the present invention provides a transformer design that maintains these characteristics and provides additional benefits.
Another challenging task is matching the relatively high output impedance (i.e. typically 50 ohms) of the electronic oscillator circuitry feeding into the relatively low input impedance (i.e. typically several ohms) of the input to the first stage of the RF power amplifier chain. The present invention can also be used to address this challenge.
The most mature RF impedance matching transformer technology is the use of wire wound on ferrite cores. This technology dates back to the middle 1950's and is commonly used at lower RF frequencies (i.e. below 80 MHz) as ferrite transformers tend to be lossey at higher RF frequencies (i.e. above 80 MHz). At high RF power levels (say, above 300 W) and for frequencies above 80 MHz, the loss within the ferrite creates a thermal problem and, therefore, adds further design complexity and cost to the RF supply. Many users of CO2 lasers having up to approximately 100 W of output power usually desire to have the laser's RF power supply mounted directly on the laser head. The totally self-contained laser and power supply allows the user to avoid dealing with a co-axial cable connecting the laser head to a remotely located RF power supply. This desire is especially strong in applications that require the laser to be mounted on a robotic arm.
Cooling the ferrite transformers within RF power supplies mounted directly on laser heads is especially difficult when air cooling is desired. In addition to the loss at higher RF frequencies, ferrite transformers tend to have larger height, width and depth dimensions than other components on the printed circuit board (PCB).
The electrical characteristics of ferrite transformers vary from unit to unit so as to require special sorting before being used in a PCB assembly. The sorting results in special tuning steps required during power supply assembly. The sorting, assembly tuning and thermal management raise the final cost of the laser and are major disadvantages of this technology.
FIG. 1A schematically shows a co-axial cable 100 that includes an outer conductor 102 and an inner conductor 104 with a dielectric 106 separating the two conductors. FIG. 1B is a schematic illustration of a 1-to-4 step-up co-axial transformer frequently used in RF power supplies to drive sealed-off, diffusion cooled CO2 laser discharges. Such transformers are presently used in commercially available 100 MHz power supplies driving 20 W to 100 W CO2 wave-guide laser discharges. FIG. 1C shows a schematic of the physical electrical connections between two co-axial cables of equal length L to form the 1-to-4 step-up transformers shown in FIG. 1B.
The advantages of the co-axial transformers over the ferrite transformer approach are lower cost, lower RF losses and the capability of higher frequency operation. Unfortunately, the co-axial transformer technology shares some of the same disadvantages associated with ferrite transformer technology. These disadvantages are: the need to mount, restrain, and connect the transformer onto the PCB; the completed transformer has a relatively large height dimension when compared to the other components on the PCB; and its electrical characteristics are strongly related to position and manner of connection to the PCB. The last issue is the one of most concern.